Significance

Mechanics is the original discipline of
physics and was formerly known as natural philosophy, dealing with
forces and motion in the macroscopic world as the human eye
perceives it. It has developed into a huge body of knowledge about
important aspects of the natural world. Modern mechanics
encompasses the movement of all matter in the universe under the
four fundamental
interactions (or forces): gravity, the strong
and weak
interactions, and the electromagnetic
interaction.

Classical versus quantum

Historically, classical mechanics came first,
while quantum mechanics is a comparatively recent invention.
Classical mechanics originated with Isaac
Newton's Laws of
motion in Principia
Mathematica, while quantum mechanics didn't appear until 1900.
Both are commonly held to constitute the most certain knowledge
that exists about physical nature. Classical mechanics has
especially often been viewed as a model for other so-called
exact
sciences. Essential in this respect is the relentless use of
mathematics in
theories, as well as the decisive role played by experiment in generating and
testing them.

Quantum mechanics
is of a wider scope, as it encompasses classical mechanics as a
sub-discipline which applies under certain restricted
circumstances. According to the correspondence
principle, there is no contradiction or conflict between the
two subjects, each simply pertains to specific situations. Quantum
mechanics has superseded classical mechanics at foundational level
and is indispensable for the explanation and prediction of
processes at molecular and (sub)atomic level. However, for
macroscopical processes classical mechanics is able to solve
problems which are unmanageably difficult in quantum mechanics and
hence remains useful and well used.

Einsteinian versus Newtonian

Analogous to the quantum
versus classical reformation, Einstein's
general
and special
theories of relativity
have expanded the scope of mechanics beyond the mechanics of
Newton and
Galileo,
and made small corrections to them. Relativistic corrections were
also needed for quantum mechanics, although relativity is
categorized as a classical theory.

There are no contradictions or conflicts between
the two, so long as the specific circumstances are carefully kept
in mind. Just as one could, in the loosest possible sense,
characterize classical mechanics as dealing with "large" bodies
(such as engine parts), and quantum mechanics with "small" ones
(such as particles),
it could be said that relativistic mechanics deals with "fast"
bodies, and non-relativistic mechanics with "slow" ones. However,
"fast" and "slow" are subjective concepts, depending on the state
of motion of the observer. This means that
all mechanics, whether classical or quantum, potentially needs to
be described relativistically. On the other hand, as an observer,
one may frequently arrange the situation in such a way that this is
not really required.

Other distinctions between the various
sub-disciplines of mechanics, concern the nature of the bodies
being described. Particles are bodies with little (known) internal
structure, treated as mathematical points in classical mechanics.
Rigid bodies have size and shape, but retain a simplicity close to
that of the particle, adding just a few so-called
degrees of freedom, such as orientation in space.

Otherwise, bodies may be semi-rigid, i.e.
elastic,
or non-rigid, i.e. fluid.
These subjects have both classical and quantum divisions of
study.

For instance: The motion of a spacecraft,
regarding its orbit and
attitude (rotation), is described by the
relativistic theory of classical mechanics. While analogous motions
of an atomic
nucleus are described by quantum mechanics.

Sub-disciplines in mechanics

The following are two lists of
various subjects that are studied in mechanics.

Note that there is also the "theory
of fields" which constitutes a separate discipline in physics,
formally treated as distinct from mechanics, whether classical
fields or quantum
fields. But in actual practice, subjects belonging to mechanics
and fields are closely interwoven. Thus, for instance, forces that
act on particles are frequently derived from fields (electromagnetic or
gravitational),
and particles generate fields by acting as sources. In fact, in
quantum mechanics, particles themselves are fields, as described
theoretically by the wave
function.